It's OK. This thread has given me plenty of ideas. The player was absent for a few sessions, so I had Halaster teleport him away "to give the monsters a fighting chance". When the player returned, I said that his PC felt like he'd just been dreaming but that he'd been having a nightmare filled with floating diamond lozenge thingies and spider-like creatures. Now, anytime he uses his annoying tanky
spirit guardians + Dodge combo, Halaster is going to have a Strixhaven mage hunter with the Mage Slayer feat show up to try and shake him out of his one trick poniness.
Currently, they're on a level that's a big, trap-filled obstacle course without many monsters to fight, but the main boss of the level is a death tyrant with a horde of zombies (seriously, there are like 40 of them, it's going to be a nightmare to manage at the table! I'll probably have to send them in in waves). He'll probably use his
spirit guardians trick then, and so things will get more complicated when a mage hunter shows up too.
Elsewhere in the dungeon, Halaster has trapped a dwarf vampire in a coffin with a magic rune keeping him in there. The coffin's at the bottom of a pit. The first person to cross over the pit gets zapped with the rune (no save), which is a "death mark". The coffin then disintegrates and the vampire is free to hunt down the person with the death mark on them. Since the obstacle course is full of teleport traps designed to split the party, I'm hoping Mr One Trick Pony will end up triggering the vampire all by himself.